1. Field of Invention
The present invention relates to an engine starting device and an engine starting method for a hybrid motor vehicle that include an electric differential, which includes a differential mechanism and an electric motor. The engine starting device and the engine starting method also include a transmission provided in a power transmission path that extends from the differential to the driven wheels; and more particularly, to an engine starting device that operates when an electric motor is malfunctioning.
2. Description of the Related Art
A powertrain for a hybrid motor vehicle includes a differential that includes a differential mechanism including a first rotating element connected to an engine, a second rotating element connected to a first electric motor and a third rotating element connected to both a second electric motor and a transfer member, the differential distributes the output power of the engine to the first electric motor and the transfer member, and a transmission is provided in a power transmission path that extends from the transfer member to the driven wheels. In this powertrain, there is known a control device that increases the rotational speed of engine to an engine starting speed (ignitable speed) or more when starting the engine by using the first or second electric motor.
For example, Japanese Patent Application Publication No. JP-2005-264762 describes a control device for a motor vehicle drive unit. The control device is used in a transmission mechanism that includes a differential whose differential mechanism includes of planetary gear sets, and a transmission that consists of a stepped automatic transmission in which a power transmission path is selectively converted between a power transfer state and a power interrupting state by engagement operations of hydraulically-operated frictional coupling devices. When starting an engine with the transmission is in “P (Parking)” or neutral “N (Neutral)”, which keeps the power transmission path in the power interrupting state, the control device increases the respective rotational speeds of a second rotating element and a third rotating element by using the first and second electric motors, thereby ensuring that the rotational speed of the first rotating element, i.e., the rotational speed of the engine connected to the first rotating element, is rapidly increased to at least an engine starting speed in accordance with the relationship of the relative rotational speeds between the first, second and third rotating elements.
If the second electric motor malfunctions or fails in the motor vehicle drive unit described in Japanese Patent Application Publication No. JP-2005-264762, however, no reaction force is generated by the second electric motor connected to the third rotating element. Even if the first electric motor is driven in this situation, the third rotating element, which is connected to the second electric motor, is rotated in a reverse direction. For instance, while a motor vehicle waits for a stoplight to change, i.e., when the vehicle speed is zero while a shift lever is in a “D (Drive)” position, the engine is typically stopped to reduce fuel consumption. If the second electric motor malfunctions or fails when the engine is started again, there is a need to adopt a reaction force generating method that does not resort to the second electric motor. It would be conceivable, as an alternative reaction force generating method, to generate a reaction force by bringing the power transmission path extending from a power transfer member to driven wheels into a power transferable state and connecting the third rotating element to the driven wheels through the transmission. However, this may reverse the rotation of the third rotating element when the shift lever is in the “D” position. The reason is that, when the shift lever is in the “D” position, there no lock mechanism is provided, unlike the lock that serves to mechanically lock the output shaft when the shift lever position is in “P” position. Accordingly, if the second electric motor malfunctions or fails when the shift lever is not in “P” position, it may be impossible to start the engine.